The number of connection holes for connection between different wiring layers is determined according to the widths of the wirings to be connected and the current to be passed through the wiring formed in the connection holes. However, in the case where a large-width wiring (for example, a wiring not less than 1 μm in width) and a small-width wiring (for example, a wiring about 0.14 μm in width) are connected, for example, where a small-width wiring is led out from a power supply wire for potential fixation, a single connection hole is used for the connection.
The material for forming the wiring layers and the material used for insulation between the wiring layers have come to changed to a low-resistance wiring material represented by copper (Cu) and to a low-dielectric-constant insulating material represented by polyaryl ether-based resins (for example, FLARE produced by Allied Signal, SiLK produced by Dow Chemical, VE produced by Schumacher, and the like are known), silicon oxycarbide (SiOC) and the like, for coping with the wiring delay arising from scale-down of wiring pitch.
In addition, it has been reported that Cu as a low-resistance material is more excellent in electro-migration resistance than aluminum (Al) which has been widely used as a wiring material. In the case of forming a small-width wiring by use of Cu, however, it is difficult to use a dry etching method, since there has not been found any dry etching gas suitable for etching Cu at a high selectivity ratio relative to the insulating film serving as a base material. Therefore, it is a general practice to form a buried wiring by a trench wiring method (for example, the Damascene process). Particularly, a method of filling (or burying) connection holes and a wiring layer simultaneously (for example, the dual Damascene process) is considered to be a promising method from the viewpoints of enlarging the register margin in lithography and shortening the processing steps.
As has been mentioned above, the wiring material has been changed from Al to Cu. As shown in FIG. 12, by using a pattern in which a single connection hole 125 was provided for connecting a small-width wiring 123 to a large-width wiring 121 satisfying the relationship of (wiring width)/(connection hole diameter)≧7 and a single connection hole 126 was provided for connecting a small-width wiring 124 to a large-width wiring 122 satisfying the relationship of (wiring width)/(connection hole diameter)≧7, a high-temperature standing test (the specimen was left to stand at 225° C. for 500 hr) as a wiring reliability evaluation was carried out.
As a result of analysis of a defective portion upon the test, losing of Cu was confirmed on the side of the large-width wiring 121 beneath the connection hole 125, as shown in FIG. 13. In addition, as shown in FIG. 14, a similar phenomenon was confirmed also in the case where the large-width wiring 121 was disposed on the upper side of the connection hole 125; in this case, losing of Cu was generated in the inside of the connection hole 125. The mechanism of these defects has not yet been elucidated. It is supposed, however, that under the influences of a stress due to a difference in thermal expansion coefficient between the wiring and the insulating film and a stress of the insulating film itself, migration of Cu was caused at the wiring beneath the connection hole, resulting in the losing of Cu. Besides, in view of the fact that dependency on wiring width is seen, it is considered that the phenomenon was caused also by the influence of volumetric shrinkage attendant on crystal growth at the wiring.